The present invention generally relates to a method and system for monitoring the condition of bearings mounted on a rotating shaft where the monitoring sensor is positioned remotely from the bearings. The present invention particularly relates to a method and system for remotely monitoring the condition of differential bearings mounted on rotating shaft in an aircraft gas turbine engine for the purpose of detecting defects therein prior to the point of bearing failure.
Rolling element bearings are used to facilitate rotation of a shaft relative to a stationary or rotating support, with minimal friction. They are typically comprised of concentric inner and outer races between which are positioned rolling elements. The rolling element can be a spherical ball (in the case of ball bearings) or a cylindrical roller element (in the case of roller bearings). Ball bearings are capable of supporting both radial and axial shaft loads, while roller bearings support radial loads only. The primary shaft race rotates synchronously with the shaft. In static housing bearings, the second race does not rotate and is supported by a stationary housing. In the case of intershaft or differential bearings, the second race rotates synchronously with a secondary shaft. Utilization of differential bearings can result in the advantages of significantly reduced system size and weight.
Like any other mechanical part, bearings can fail due to wear, lack of lubrication, contamination, assembly damage, excessive load or other factors. As a result, the monitoring of the condition of bearings has received considerable attention for some time because bearing failures can be catastrophic, leading to significant collateral damage and expensive repair, in addition to the time the machine is out of service. Monitoring of bearings to determine when they have or are about to become faulty can be more difficult when multiple bearing assemblies are being used in the machine, which is typical in gas turbine engines. Monitoring of bearing condition can be further complicated by other factors in the machine, such as high temperatures, difficulties in locating the monitoring sensors in the machine, other sources of vibration, and the like. When used with aircraft engines, the monitoring system also needs to be relatively lightweight.
An example of a gas turbine engine having multiple bearing assemblies is shown in U.S. Pat. No. 5,749,660 (Dusserre-Telmon et al), issued May 12, 1998. The turbine engine shown in FIG. 1 of the Dusserre-Telmon et al patent has six bearing assemblies (5 through 10) associated with a rotating shaft having a coaxial front part 1 and a rear part 2. These bearing assemblies include the combination of a ball bearing 5 and roller bearing 6 that support front part 1 of the shaft at the forward end of the engine, a pair of ball bearings 7 and 8 that support one end of the rear part 2 of the shaft and a pair of roller bearings 9 and 10 at the aft portion of the engine that support the other end of the rear part of 2 of the shaft.
In FIG. 1 of the Dusserre-Telmon et al patent, roller bearing 9 is shown in the form of an intershaft or differential roller bearing where both the inner and outer races are rotating. It has been found that for some aircraft engine models, intershaft bearing failures, i.e., at the position of roller bearing 9, can lead to aircraft engine in-flight shutdowns. A common failure mode for rolling element bearings of this type is localized defects, in which a sizable piece of the contact surface is dislodged during operation, mostly by fatigue cracking in the bearing metal under cyclic contact stressing. The monitoring of potential failure of such roller element bearings is thus often based on the detection of the onset of such localized defects.
One method for monitoring for such localized defects is to examine the debris present in the lubricant used in the bearing. Gas turbine engines typically have metal chip detectors (MCD) installed in the engine oil scavenge system. The MCD""s collect metal debris that is transported in the lubricating oil; this collected metal debris can then be examined to determine if bearing material is present. Unfortunately, debris analysis can be unreliable for detecting defects in intershaft bearings because the bearing debris can be trapped inside the rotor by centrifugal forces and thus remain undetected by an MCD.
Another method for monitoring such localized defects is by vibration analysis. During bearing operation, bursts of acoustic emissions or vibrations result from the passage of the defect through the roller and raceway contacts. Defects at different locations of a bearing (inner race, roller and outer race) will have characteristic frequencies at which the bursts are generated. Theoretical estimations of these frequencies are called characteristic defect frequencies. Therefore, the signal of a damaged bearing (hereafter referred to as the xe2x80x9cbearing defect peakxe2x80x9d) will typically consist of a periodic burst of acoustic emissions or vibrations near or about the characteristic defect frequency. In addition, the changing amplitude of this bearing defect peak over time can be used to quantify the degree of bearing failure that has occurred, preferably sufficiently in advance so that maintenance and repair can occur before there is total bearing failure. Unfortunately, the characteristic frequencies are usually sufficiently high that they can attenuate rapidly in the surrounding structures. For this reason, it is generally desirable to locate vibration sensors as close to the bearing as possible.
A particular problem in gas turbine engines exists when an intershaft or differential bearing, like bearing 9 in the Dusserre-Telmon et al patent, is positioned in a high temperature section of the engine. This makes it extremely difficult or potentially impossible to position a reliable vibration sensor close to this bearing so that it will survive and function in a high temperature environment. As a result, in the high temperature sections of the engine, the nearest practical location where sensors can survive and function are on the exterior of the engine. However, due to their high frequencies, characteristic bearing defect signals are usually attenuated before they can reach external engine sensor locations, meaning the defect will typically remain undetected.
Isolating the characteristic bearing defect frequency and amplitude from other acoustic emissions can also be difficult. Usually, the vibration sensor signal contains broadband frequency content (i.e., numerous frequencies across a broad frequency range); within this content is the characteristic bearing defect frequency which co-mingles with these other frequencies. The bearing defect peak at the characteristic frequency also does not necessarily have the highest amplitude, and is not typically self-evident. In addition, other sources of vibration can reside at the same frequency as the characteristic bearing defect frequency, which could lead to false detection events. For example, the signal can potentially be an aberration due to fluctuations in the rotational speed of the shaft, as well as the inner and outer races.
Accordingly, it would be desirable to provide a method and system for remotely monitoring the condition of bearings mounted on a rotating shaft, especially one used in a gas turbine engine, where multiple bearing assemblies are present, that can reliably detect and isolate the signal of the characteristic frequency of bearing failure of interest from a broadband signal, without invasive analysis techniques, and is relatively lightweight for use with aircraft engines.
The present invention relates to a method and system for monitoring the condition of a bearing mounted on a rotating shaft, particularly one present in a gas turbine engine of an aircraft, where the vibration sensor that monitors the bearing is remote therefrom but proximate to the rotating shaft. The method of the present invention comprises the steps of and the system of the present invention is capable of:
a. obtaining through the vibration sensor a broadband signal having frequencies that include the bearing defect peak of the monitored bearing;
b. analyzing the broadband signal to identify the presence of the bearing defect peak; and
c. if the bearing defect peak is present, quantifying the amplitude of this peak to determine whether degradation of the monitored bearing has at least reached a threshold criteria previously established.
The method and system of the present invention provides a number of benefits and advantages in monitoring bearings mounted on a rotating shaft, especially one used in an aircraft gas turbine engine. The method and system of the present invention allows for reliable detection, isolation, identification and quantification of the bearing failure signal for the bearing of interest that is being monitored, even when multiple bearing assemblies are mounted on the rotating shaft. The method and system of the present invention provides a non-invasive analytical technique so that the bearing being monitored does not have to be removed to carry out the analysis. Because the vibration sensor can be mounted in the forward, lower temperature end of a gas turbine engine, the method and system is particularly useful in monitoring intershaft or differential bearings mounted at the aft, higher temperature end, of the gas turbine engine. The system of the present invention is also relatively lightweight, making it ideal for monitoring bearings in aircraft gas turbine engines.
The method and system of the present invention takes advantage of an alternate, less obvious transmission path for monitoring the condition of bearings mounted on a rotating shaft, especially one used in an aircraft gas turbine engine. It has been found that when the vibration sensor can be practically located proximate or near the shaft, with a relatively low mass between the sensor and the bearing being monitored, the characteristic defect frequency can be detected, even when the sensor is relatively remote from the bearing being monitored. In particular, the vibration sensor can be located in a lower temperature section of a gas turbine engine, yet detect a defect in a bearing located remotely therefrom in the higher temperature section of the engine when: (1) the vibration sensor is positioned proximate to the shaft; and (2) the portion of the shaft between the sensor and the bearing being monitored is of relatively low mass so that acoustic vibrations from the bearing will be transmitted by the shaft and picked up by the sensor.